Thermal dehydration of aliphatic acids



Patented June 6, 1939 PATENT OFFICE 2,160,842 THERMAL DEHYDRATION 0F ALIPHATIC' I Y ACIDS I Y 5 Henry Dreyfus, London, England No Drawing.

7 Claims.

This invention relates to improvements in the thermal dehydration of aliphatic'acids, and is more particularly'concerned with the manufacture of acetic anhydride or ketene by the thermal dehydration of acetic acid.

The general method of manufacturing acetic anhydride or kete'ne directly from acetic acid is to subject the acetic acid to thermal decomposition or dehydration, usually in the presence of suitable catalysts. While dehydration takes place over a wide range of temperature, the most useful range is generally 500-1000 C. and especially GOO-900 C., though of course the 'optimum temperature naturally depends upon the particular conditions employed and notably the particular catalyst and the pressure or partial pressure of the acetic acid. In U. S. Patent No. 1,735,962 I have described catalysing the reaction by means of a phosphoric acid and have further indicated that the process may be carried out under reduced pressure or at ordinary pressure. Further in U. S. Patent No. 1,883,353 I have shown the advantages of carrying out the process in the presence of bases, and in particular ammonia and the organic bases which are substitution products of ammonia, for example pyridine, piperidine, aniline, alkyl anilines, toluidines and the like. Again in this patent it is indicated that while the reaction may be carried out at pressures higher than atmospheric, it is preferable to employ atmospheric pressure or reduced pressure.

According to the present invention, the thermal dehydration of acetic acid or other aliphatic acid is catalysed by means of partially amidated oxygen acids of phosphorus or by means of esters of such partially amidated acids. My U. S. application S. No. 152,761 filed July 9, 1937, corresponding to British application No. 19860/36 filed July 17, 1936, describes the use for the same purpose of the completely amidated products. The basic acids which are partially amidated or partially amidated and partially esterified for use in accordance with the present invention may, for example, be phosphorus and phosphoric acids. Thus the substances to be used in accordance with the present invention may have the general formula where one or two of the radicles X, Y and Z are of the form NRlR2, where R1 and R2 are hydrogen or an organic radicle which may be aliphatic,

Application July 9, In Great Britain July 17,

1937, Serial N0.

aromatic or hydro-aromatic, and the remaining X, Y and Z radicles are of the form 0R3, where R3 is hydrogen or an organic acid radicle which may, for example, be aliphatic, aromatic or by dro-aromatic. RIRZ may jointly represent an alkyl'en'e residue, so that the amide grouping is, for example, the residue of piperidine. I prefer to employ the derivatives of. phosphoric acids and, moreover, to employ compounds which are of relatively high boiling point.

' In the above formulae R1, R2 and R3 may, for example, be methyl, ethyl, propyl, isobutyl, phen- 571 or substituted phenyl. As examples of compounds of the phosphoric acid class, I may mention phosphoric acid mono-amide mono-diethylamide or -anilide; phosphoric acid mono-ethyl ester mono-amide mono-anilide; phosphoric acid mono-ethyl ester bis-diethylamide, bis-anilide or bis-piperidine; phosphoric acid mono-ethyl ester diethylamide piperidine, mono-ethyl ester monoanilide, mono-phenyl ester monoor bis-anilide, monol-chlorphenyl ester bis-anilide, diethyl ester diethylamide, diethyl ester methylanilide, diethyl ester 4-chloranilide or para-toluidide, diphenyl ester methyl anilide; phosphoric acid bis-- 4-chloranilide or bis-paraphenetidide; and among the phosphorous bodies may be mentioned phosphorous acid mono-ethyl ester bis-diethyl amide or bis-dipropylamide or bis-piperidide.

The catalysts of the invention may, if desired, be used together with nitrogenous bases, including ammonia and the organic nitrogenous bases, as referred to in U. S. Patent No. 1,883,353. The amides may be dissolved in the main bulk of the acetic acid to be subjected to thermal dehydration, or they may be injected into. the reaction chamber separately from the acid to be dehydrated, or separately from the main bulk of the acid. Most conveniently the acid is vaporised, preheated and fed to the reaction chamber and the catalyst is introduced into the superheated acid vapor in the form of a solution in a solvent which exercises no deleterious effect upon the reaction. Preferably the catalyst is dissolved in a portion of the acetic acid or other acid to be dehydrated but other solvents may be used, for instance there may be used for this purpose a quantity of a liquid which is subsequently to be employed to facilitate the separation. of the products of the thermal dehydration, as, for example, by forming an azeotropic mixture with water as referred to below. It is preferred to raise the temperature of the acid vapor by the preheating step to within 20 to C. of the temperature at which the thermal dehydration is to be effected,

for instance when employing temperatures of 670720 C. in the reaction zone the preheater may be run at 600-650 C. If desired, however, the vapor may only be heated to Within say 100 C. of the dehydration temperature or may be heated right up to the dehydration temperature.

The quantity of amide to be employed may be quite small in relation to the amount of acetic acid or other acid vapor. For example, it may be of the order of 5% of the weight of the acetic acid, but is preferably much less, say less than 1%, quantities of the order of .2 to .5% being quite sufiicient. Where a nitrogenous base is employed in conjunction with the amide, its proportion may be similar to that of the amide. base may be introduced together or separately. 7 7

The reaction is best carried out at atmospheric pressure or reduced pressure. As already indicated, the main products of the thermal dehydration of acetic acid are acetic anhydride and ketene. The anhydride or ketene may be recovered from the reaction products in any suitable manner. For example the reaction products may be cooled so as to condense the whole of the anhydride, water and unchanged acid and the ketene allowed to pass on, or the reaction products may be subjected to a fractionation treatment designed to separate the anhydride from the water and the ketene. In the latter case, it is of advantage to employ liquids such as benzene, toluene, xylene, chlorbenzene, chloroform and the like which form an azeotropic mixture with the water. The invention is particularly useful in the thermal dehydration of acetic acid to acetic anhydride and/or ketene, but it may also be applied to the thermal dehydration of other aliphatic acids, for example propionic acid, butyric acid and the like. Having described my invention, what I desire to secure by Letters Patent is:

1. Process for the thermal dehydration of an aliphatic acid, which comprises subjecting the vapor of the aliphatic acid to the action of heat with the amide 'Derature and thereafter the thermal dehydration The nitrogenous in the presence of a compound selected from the group consisting of the partially amidated oxygen acids of phosphorus and their esters.

2. Process for the thermal dehydration of an aliphatic acid, which comprises subjecting the vapor of the aliphatic acid to the action of heat in the presence of a compound selected from the group consisting of the partially amidated phosphoric acids and their esters.

3. Process according to claim 1, wherein the acid vapor is preheated to above its critical temsuperheated acid vareaction zone in which the takes place and said compound is introduced into the superheated acid Vapor in the form of a solution.

4. Process for the thermal dehydration of acetic acid, which comprises subjecting acetic acid vapor to the action of heat in the presence of a compound selected from the group consisting of the partially amidated oxygen acids of phosphorus and their esters.

5. Process for the thermal dehydration of acetic acid, which comprises subjecting acetic acid vapor to the action of heat in the presence of a compound selected from the group consisting of partially amidated phosphoric acids and their esters. a

6. Process according to claim 4, wherein the acid vapor is preheated to above its critical temperature and thereafter the superheated acid vapor is passed into a reaction zone in which the thermal dehydration takes place and said compound is introduced into the superheated acid vapor in the form of a solution.

7. Process according toaclaim 5, wherein the acid vapor is preheated to above its critical temperature and thereafter the superheated acid Vapor is passed into a reaction zone in which the thermal dehydration takes place and said compound is introduced into the superheated acid vapor in the form of a solution.

' HENRY DREYFUS.

por is passed into a 

